The myocardium is susceptible to mechanical stimulation: case reports have detailed incidents of cardiac arrest due to a ball or a fist hitting the person's thorax. Physiological studies have reported initiation of an action potential due to a mechanical stimulation such as tapping of the epicardium [Avitall, B., Levine, H. J., Naimi, S., Donahue, R. P., Pauker, S. G., and Adam, D. R., “Local effects of electrical and mechanical stimulation on the recovery properties of canine ventricle,” Am. J. Cardiology 50, 263-270 (1982)], or due to stretch. Separately, high energy ultrasound pulses have been reported to shatter kidney stones, while also affecting inner cellular structures and membrane properties. In particular, it has been reported that ultrasound pulses increase membrane permeability and specific ion flow. Ultrasound transducers attached to cardiac catheters have been used to induce electrical activity locally in the heart, for diagnostic purposes. External ultrasound transducers used for imaging have also been observed to induce action potentials in the heart, inadvertently, especially when contrast agents containing small bubbles are injected into the bloodstream. Because of the danger of inducing action potentials in the heart in an uncontrolled way, great care is taken during ultrasound imaging of the heart not to induce action potentials.
Direct electrical stimulation of the heart is done for cardiac pacing, usually via electrodes implanted during surgery. Electrical stimulation is also used to measure cardiac response, as a diagnostic, and for this purpose electrodes may be inserted by catheters via the venous or arterial system. Catheters are also used to carry laser or RF or ultrasound energy to specific sites in the heart or elsewhere in the circulatory system, in order to ablate tissue for therapeutic purposes. All of these invasive procedures involve obvious risks and expenses.
Ultrasound energy from external transducer arrays has been focused on tumors in order to destroy them by heating.